The present invention relates to a light power modulating system employed in a scanning optical system for a laser beam printer.
A scanning optical system, which is employed, for example, in a laser beam printer, is provided with a light power modulating circuit for modulating a laser beam directed to a surface of a photoconductive drum. The light power modulating circuit modulates the laser beam, which is emitted by the laser diode, according to an image signal transmitted from an external device to form a latent image on the photoconductive drum.
However, it is known that an output power of the laser diodes varies with temperature and/or time. In addition, characteristics of an output power differ among laser diodes even though the specification is the same.
In order to avoid individual differences of the output power of the laser diodes due to causes described above, the light power modulating circuit generally has a function of an automatic power controller (APC). In the light power modulating circuit, an output power of the laser diode is monitored using a photodiode, and then the electrical current supplied to the laser diode is adjusted based on the monitored output power of the laser diode.
FIG. 1 schematically shows a circuit diagram of the light power modulating circuit having the APC function. As shown in FIG. 1, image data Sdata is input to a D/A converter 21, which converts the image data (Sdata) to an analog image signal (Vdata). This analog image signal (Vdata) is input to a positive input terminal of a differential amplifier 22.
The differential amplifier 22 outputs a differential amplification signal Vdef representing an amplified difference between the input analog image signal (Vdata) and a monitor signal (Vmon) corresponding to the current output power of the laser beam. The differential amplification signal Vdef is input to a voltage-to-current (V-I) converter 23.
The voltage-to-current (V-I) converter 23 generates a driving current (Id) responsive to the amplitude of the differential amplification signal Vdef. Then, a laser diode LD emits a laser beam whose intensity corresponds to the driving current Id.
When the laser diode LD emits the laser beam, for example, to perform a scanning operation, a backwardly emitted laser beam (a back beam) is received by a photodiode PD, which generates a monitor current (Imon) responsive to the intensity of the back beam. It should be noted that the intensity of the back beam is proportional to that of the normal (i.e., the forwardly emitted) laser beam used for the scanning operation. The monitor current (Imon) is input to a current-to-voltage (I-V) converter 24, which generates a monitor voltage (Vmon) corresponding to the monitor current (Imon).
As shown in FIG. 1, the monitor voltage Vmon is applied to a negative input terminal of the differential amplifier 22, while the analog data singal Vdata is input to the positive input terminal of the differential amplifier 22.
If the output power of the laser diode LD decreases due to, for example, deterioration with time, the monitor voltage Vmon decreases, thereby the differential amplification signal (Vdef) increases. As the differential amplification signal (Vdef) increases, the driving current (Id) increases, thereby the output power of the laser diode LD also increases.
If the output power increases, on the contrary, due to, for example, thermal variations, then the monitor voltage (Vmon) increases, thereby the differential amplification signal (Vdef) decreases. As the differential amplification signal (Vdef) decreases, the driving current (Id) decreases, thereby the output power of the laser diode LD also decreases.
In the light power modulating circuit shown in FIG. 1, the output power of the laser diode LD (i.e., the driving current Id) is controlled such that the monitor voltage Vmon coincides with the voltage Vdata. In other words, the automatic power control is performed.
It should be noted that, if the analog data signal Vdata applied to the positive input terminal of the differential amplifier 22 varies as the image signal Sdata varies, the driving current (Id) varies accordingly. Thus, the output power of the laser diode LD is modulated, using the APC function, in accordance with the:image data (Sdata).
In the light power modulating circuit shown in FIG. 1, when a response of the V-I converter 23 or the I-V converter 24 is not sufficiently fast, if the analog signal (Vdata) increases rapidly, the differential amplification signal (Vdef) also must increase rapidly. Furthermore, due to slow response of the V-I converter 23 or the I-V converter 24, the monitor voltage Vmon does not increase immediately. Then, the differential amplification signal Vref remains relatively large for a certain period of time. This will cause the V-I converter 23 to generate a surge current which may cause destruction of the laser diode LD. If the analog data signal (Vdata) decreases rapidly and the differential amplification signal (Vdef) decreases rapidly, the driving current (Id) generated by the V-I converter 23 decreases significantly. In such a case, the output power of the laser diode drops significantly, and becomes lower than necessary.
For this reason, a feedback time that is defined as a time period, within which the monitor voltage Vmon reflecting the change of the analog data signal Vdata is input to the differential amplifier 22, should be relatively short. In order to shorten the feedback time, the differential amplifier 22, the V-I converter 23, and I-V converter 24 must be constructed with quick response type electronic parts. However, such quick response type parts are expensive.
In addition, the feedback time is a sum of the response times of the differential amplifier 22, V-I converter 23, and I-V converter 24. Accordingly, it has been difficult to decrease the feedback time sufficiently in the light power modulating system constructed as above.
It is therefore an object of the invention to provide an improved light power modulating system capable of preventing the above-described deficiencies of the light power modulating system.
For the above object, according to the invention, there is provided a light power modulating system for modulating output power of a laser diode. The light power modulating system is provided with a monitoring system that detects an output power of the laser diode and outputs a. voltage signal corresponding to the detected output power, an image signal output system that outputs an image signal representative of a thickness density of an image to be: formed, a first modifying system that modifies the image signal and outputs a modified image signal, a changing rate of the modified image signal between a first lower value and a first higher value being lower than that of the image signal, a differential amplifier that outputs an amplified difference between the modified image signal and the voltage signal output by the monitoring system, and a driving current generating system that generates a driving current for the laser diode based on the amplified difference output by the differential amplifier.
In particular, the first modifying system may include a high frequency attenuator.
In this case, the high frequency attenuator may include an active filter which reduces higher frequency signals more than lower frequency signals. Alternatively, the high frequency attenuator may include a passive filter which reduces higher frequency signals more than lower frequency signals.
Optionally, the detecting system may include a photodiode that receives the laser beam emitted by the laser diode and generates an electrical current corresponding to the intensity of the received laser beam. The detecting system may also include a current-to-voltage converter that converts the electrical current generated by the photodiode to a voltage corresponding to the electrical current.
Still optionally, the image signal output system may receive a digital signal representing the thickness density of the image to be formed and generates the image signal as an analog signal. An amplitude of the image signal corresponds to the thickness density of the image to be formed.
According to another aspect of the invention, there is provided a light power modulating system for modulating output power of a laser diode. The light power modulating system is provided with a monitoring system that detects an output power of the laser diode and outputs a voltage signal corresponding to the detected output power, an image signal output system that outputs an image signal representative of a thickness density of an image to be formed, a second modifying system that modifies the voltage signal and outputs a modified voltage signal, a changing rate of the modified voltage signal between a second lower value and a second higher value being higher than that of the voltage signal, a differential amplifier that outputs an amplified difference between the image. signal and the modified voltage signal output by the monitoring system, and a driving current generating system that generates a driving current for the laser diode based on the second amplified difference output by the differential amplifier.
Optionally, the light power modulating system may include a third modifying system that lowers a changing rate of the amplified difference between a third lower value and a third higher value.
In particular, the third modifying system may include a high frequency attenuator.
Still optionally, said second modifying system may include a high frequency booster.
In this case, the high frequency booster may include an active filter which boosts higher frequency signals more than lower frequency signals. Alternatively, the high frequency booster may be provided with a passive filter which boosts higher frequency signals more than lower frequency signals.
Further optionally, the detecting system may include a photodiode that receives the laser beam emitted by the laser diode and generates an electrical current corresponding to the intensity of the received laser beam. The detecting system may also include a current-to-voltage converter that converts the electrical current generated by the photodiode to a voltage corresponding to the electrical current.
Still optionally, the image signal output system may receive a digital signal representing the thickness density of the image to be formed and generate the image signal as an analog signal. An amplitude of the image signal corresponds to the thickness density of the image to be formed.